1. Field of the Invention
The present invention relates to an inkjet printing apparatus configured to perform printing by ejecting liquid onto a printing medium.
2. Description of the Related Art
In recent years, application of inkjet printing apparatuses has been diversified into printing of photographic images on sheets large in size such as A1 size or A0 size. In such application, a large amount of ink is required for printing on a single printing medium. Meanwhile, an inkjet printing apparatus widespread in general for private use is relatively small in size, and includes an ink tank detachably or non-detachably integrated with a printing head. When using such an ink tank, a printing apparatus requires more frequent replacement of the ink tank and thus requires more time and efforts for its handling than ever before. Meanwhile, a large-capacity ink tank may be employed in order to reduce the frequency of ink tank replacement. In this case, however, the ink tank including the ink contained therein has such a large weight that power consumption required for moving the printing head is increased.
Accordingly, in an inkjet printing apparatus suitable for the above-described application, it is advantageous to employ a so-called tube supply method in which a large-capacity ink tank separate from a printing head is disposed at a fixed position on an apparatus, and ink is supplied through a tube which connects the printing head and the ink tank. Specifically, even when the large-capacity ink tank is employed, the ink tank does not have to be moved together with the printing head. Hence, it is possible to reduce the weight of the moving part and thereby to suppress power consumption at the time of printing. Moreover, being provided with a relatively large-capacity ink tank in order to perform printing, a printing apparatus using the tube supply method can perform continuous printing for a long period of time. As described above, the tube supply method allowing continuous printing for a long period of time is employed in some cases when a serial scanning type of inkjet recoding apparatus needs to be equipped with a large-capacity ink tank in order to output printed images large in size.
However, even in the case of the above-described printing apparatus employing the tube supply method, the ink tank has a limited amount of ink thereinside. Accordingly, it is necessary to replace the ink tank when the ink inside the ink tank is exhausted. Moreover, if the ink inside the ink tank is exhausted during a printing operation of a single printing medium, it is necessary to stop the printing operation and to replace the empty ink tank. In this case, the ink ejected onto the printing medium gets dried during replacement of the ink tank. As a result, when the printing operation is resumed, a color difference (unevenness of color) may appear between a printed portion formed immediately after resuming of the printing operation and the other portions. Such a color difference is apt to appear when inks in different colors are ejected onto the same position on the printing medium in an overlapping manner. Specifically, when the printing operation is performed continuously without interruption, a time gap between an ink ejected earlier and an ink ejected later is not so large. Accordingly, the ink ejected later is ejected on and thus overlaps the undried ink ejected earlier. Therefore, the inks in different colors ejected onto the same position in the overlapping manner are mixed together on the printing medium. On the other hand, if the printing operation is temporally stopped by replacement of the ink tank, the liquid ink is ejected on and thus overlaps the dried ink, and those inks are not mixed together properly. As a consequence, a portion printed immediately after replacement of the ink tank exhibits a color in which a color of either the ink ejected earlier or the ink ejected later is emphasized more. Thus, the color difference appears between the printed portion which is printed continuously and the portion where the printing is resumed after the interruption due to replacement of the ink tank.
The unevenness of color caused by interruption for replacing the ink tank as described above causes a significant adverse effect on image quality of the printing apparatus capable of large size printing at a high speed by using a long printing head, for example. Meanwhile, when a printed image cannot be used as a product due to occurrence of unevenness of color, the inks and the printing medium are wasted and running costs are thereby increased. To avoid this, Japanese Patent Laid-Open No. 2001-113716 proposes an inkjet printing apparatus using a sub tank in addition to a main tank in order to avoid a situation in which an ink tank needs to be replaced during a printing operation on a printing medium. In the inkjet printing apparatus disclosed therein, an ink is supplied from a replaceable large-capacity main tank to a relatively small-capacity sub tank, and the ink stored in the sub tank is supplied to a printing head.
Therefore, even when the ink inside the main tank is exhausted during printing on a single printing medium, ink still remains inside the sub tank, so that the printing can be continued by using the ink stored in the sub tank. Then, replacement of the main ink tank is completed while the printing is being performed by use of the ink supplied from the sub ink. Thus, the printing operation can be performed without interruption, and the high quality of the printed image can be maintained.
According to the printing apparatus disclosed in Japanese Patent Laid-Open No. 2001-113716, the printing head and the sub tank are mounted on a carriage. Moreover, the main tank is disposed in a position separate from the carriage, and an ink flow path is disposed to extend from the main tank to the sub tank. The ink flow path extending from the main tank to the sub tank is connectable to and disconnectable from the sub tank. The ink flow path extending from the main tank is provided with a pump for supplying the ink from the main tank to the sub tank.
As described above, according to the printing apparatus of Japanese Patent Laid-Open No. 2001-113716, the pump is disposed in the ink flow path between the main tank and the sub tank, and the ink is supplied from the main tank to the sub tank by use of this pump. However, the pump for supplying the ink from the main tank to the sub tank is often expensive. In general, the pump requires various structures including a driving source, a transmission mechanism for transmitting a driving force generated by the driving source, the ink flow path, and the like. For this reason, the pump is relatively costly among components included in a printing apparatus. Moreover, the printing apparatus configured to supply the ink from the main tank to the sub tank also needs an exhaust mechanism. The exhaust mechanism has to be equipped with a pump or a valve which allows the sub tank to communicate with or to be blocked from atmosphere and a driving mechanism for driving the valve, for example. Therefore, the configuration of the exhaust mechanism is likely to be complicated and costly.
Meanwhile, ink used for inkjet printing is classified broadly into an ink mainly containing a dye component as a coloring material (hereinafter referred to as a dye ink) and an ink mainly containing a pigment component (hereinafter referred to as a pigment ink). For an application that requires light resistance or gas resistance of a printed material, use of the pigment ink, particularly, is often effective in ensuring sufficient fastness of an image. However, the pigment ink has various problems in handling as compared to the dye ink. Dispersibility of the pigment component being the coloring material in the ink is one of the problems, for example. The pigment component is not dissolved in an ink solution unlike the dye component but is floating in the fluid in a dispersed state. Accordingly, if no printing operation takes place for a while, pigment particles inside the ink tank gradually settle out due to gravity, thereby causing a difference in density distribution of the pigment particles in the vertical direction of the ink tank. Specifically, a layer having a high coloring material density is formed in a lower portion while a layer having low coloring material density is formed in an upper portion. If the printing operation is started and continued in this state, a density difference occurs on an outputted image.
To solve this problem, it is effective to cause a flow or a movement of the ink inside the tank by increasing or reducing the pressure of an ink supply path such as a tube, thereby performing an agitating operation of the ink inside the ink tank. In this case, it is desirable to achieve a state of a small difference in the density distribution (a state of uniform dispersion of the coloring material) inside the tank by performing a preferable agitating operation without causing structural complication of the printing apparatus.